Add comprehensive CS1622 compiler error documentation

[Copilot]

This PR creates detailed documentation for CS1622 compiler error to address the misleading information identified in the issue. The original documentation suggested that iterator methods cannot use return statements at all, which is incorrect.

What was fixed

The CS1622 error documentation was incomplete and misleading. The current mention in iterator-yield.md states that iterator methods "can't also use a return statement to return a sequence," but this oversimplifies the actual rules and could confuse developers.

What this PR adds

Creates a new cs1622.md file that provides comprehensive coverage of:

1. Clarified rules: You cannot mix yield return with return statements in the same iterator method, but you CAN use a single return statement to return an object that implements the required interface (when not using any yield statements).

2. Clear examples showing both scenarios:

   // This causes CS1622 - mixing yield and return
   public static IEnumerable<int> Invalid()
   {
       yield return 1;
       return new[] { 2, 3 }; // CS1622
   }
   
   // This is valid - return without yield
   public static IEnumerable<int> Valid()
   {
       return new[] { 1, 2, 3 }; // OK
   }

3. IAsyncEnumerable coverage: The same rules apply to async iterator methods, which was requested in the issue but not documented anywhere.

4. Multiple fix approaches: Practical solutions including using only yield statements, only return statements, or splitting into separate methods.

The documentation has been validated through compilation testing to ensure accuracy, and passes all markdown linting requirements.

Fixes #40067.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/misc/cs1622.md	docs/csharp/misc/cs1622

[BillWagner]

closing, as this was fixed in a previous commit.